Is TV turning into the new movies? Exhibit #3,527 for the prosecution: second and third seasons of popular series like “Stranger Things” and “Big Little Lies” that play more like sequels, with all the attendant problems and potential strengths of “Blockbuster Movie, Part 2.”

Once upon a time, before cable TV and streaming video and ”The Sopranos” changed the landscape, a TV series was essentially fractal, with each episode serving as a miniature of the show as a whole. Every week we’d tune in to see Samantha Stevens use her witchy powers to save Darren’s ad-agency job, Lieutenant Columbo or Jessica Fletcher solve a murder, Mulder and Scully venture “out there” for the Truth.

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We tuned in both for variations on the theme and to be comforted with sameness, but when the circular story lines start straying too far outside the curve — when the battling lovers finally sleep together in season 2, for instance — we call that “jumping the shark” after the Fonz doing just that in season five of “Happy Days.”

Obviously, there were sequential narratives in TV, and plenty of them — think of everything from “Dallas” to “Buffy the Vampire Slayer” — but even in those cases, ensuing seasons were considered more of the same story line rather than a new chapter or a whole new book. Even breakthrough cable dramas such as “The Wire” and “The Sopranos” were conceived as single seasons-long narratives: If their various endgames weren’t in sight during the first few seasons, they all built toward a dramatic reckoning far down the road.

That multi-season-epic structure has held in place for such popular cable shows as “Game of Thrones,” “Mad Men,” and “Breaking Bad,” but they still all function as what we’ve been trained to think of as “TV,” no matter if the final chapters are mishandled (“GoT”) or done right (“Mad Men”). What has changed is the entry of streaming giants like Netflix and Amazon, testing out the waters of small-screen programming with limited-episode shows that function dramatically as one long movie and are increasingly watched as such.

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What really scrambled the formula was the practice of dumping entire seasons online at once, a practice influenced by older seasons of shows becoming available on demand through Hulu, Amazon, and other outlets. Now you didn’t have to wait a week to see the next episode of “Stranger Things.” You could just inhale all eight episodes at a gulp (or three). The relatively new phenomenon of binge-watching alters our compact with what we call “television,” in part because the streaming giants put limited series out there in their entirety without committing to follow-up seasons, and in part because those first seasons build to and resolve with dramatic climaxes that can stand on their own. Everything that follows is, by definition, a sequel.

And sequels, as we have learned from “Jaws 3-D,” “The Next Karate Kid,” and “Jurassic World,” are problematic. A telling and fairly early example was season 2 of the popular BBC crime drama “Broadchurch” in 2014. The first season’s mystery had been solved but viewers wanted to spend more time with the squabbling detectives played by David Tennant and Olivia Colman, so a new, less compelling mystery and underwritten new characters had to be created. (Charlotte Rampling is probably still cursing her agent for getting her involved.)

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Netflix’s “Stranger Things,” now in season 3, and HBO’s “Big Little Lies,” whose second season is being doled out in weekly installments, similarly struggle with the balance of telling fresh stories about old characters, as do other bravura shows like “Atlanta” (FX), “Barry” (HBO), and “Killing Eve” (BBC America). Because their primary dramatic arcs are over the course of one season, building toward a climax in eight or so episodes, each falls between the old TV series and the new open-ended serials like “Game of Thrones.” With each new season, a new arc has to be hammered out; unlike say HBO’s “True Detective” or FX’s “American Horror Story,” which reinvents itself with all-new characters each season, a new reason has to be found for audiences to follow along.

Sometimes that reason is just to follow the wayward creative genius of the people who make the shows. Season 2 of “Atlanta” allowed Donald Glover to plumb ever-more subtle layers of doubt and meaning in the black American experience, plus it gave us “Teddy Perkins,” one of the weirdest, most unsettling episodes of TV ever aired. Writer-star Phoebe Waller-Bridge used the second six-episode season of “Fleabag” (Amazon) to deepen the title character and strengthen her relationships with others and with us; the final farewell to viewers is the most emotionally moving moment I’ve seen on any screen this year.

By contrast, a lulu of a show like “Killing Eve” — on which Waller-Bridge served as writer and show-runner (but not actor) for season one — gradually tipped out of balance in its recent season 2 with Sandra Oh’s MI5 investigator behaving in increasingly illogical ways (I know, that’s the point, but it still didn’t work) and Jodie Comer’s murderous Villanelle arguably over-indulged by the writers. As sequels go, “Killing Eve 2.0” was a decidedly mixed bag.

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“Stranger Things” with Finn Wolfhard (left) and Noah Schnapp. Netflix

You might feel the same way about season 2 of HBO’s “Big Little Lies,” or the third season of “Stranger Things” on Netflix, neither of which is dramatically necessary, strictly speaking. And I would have agreed with you until recently: Each of those shows is starting to come into focus as something slightly different from what we’ve been expecting (which is, to be fair, more of the same — the curse of the audience is that we want novelty and continuity at the same time).

With each season of “Stranger Things,” the Duffer brothers essentially replay the same plot. Monsters from the Upside Down are coming and only Eleven (Millie Bobby Brown) can save us! The real hook has been watching the kid characters grow into adolescence and, in season 3, start to grow apart from one another. Structurally, that has led to four separate ongoing plot strands, featuring four separate investigations of the same phenomena, a tactic that has worked unusually well as we wait for the various lines to converge — for the characters to come together in a satisfying a-ha moment.

The show has also amped up the horror and the humor, while introducing new characters both adorable (Maya Hawke’s Robin) and annoying (Priah Ferguson’s Erica). Is it art? Nope. But I’ve got two episodes to go and I’m in for the duration.

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A second season of “Big Little Lies” has even less reason to exist — the big bad abuser of season 1 is decisively still dead — yet the show’s prime movers (writer David E. Kelley, producer Jean-Marc Vallee, director Andrea Arnold) have smartly chosen to explore the hangover of guilt and complicity among the show’s five principles and their families. To wit: A drama about the privileged classes, with a sharp, satiric eye to their excesses, is now also about the pressures all women face while juggling multiple roles as wives, mothers, lovers, and individuals.

The words “unhinged” and its corollary “hinged” have popped up a lot in the dialogue, and not just because series newcomer Meryl Streep is doing some of her finest low-key work in years as the dead man’s mousy, manipulative mother. Rather, it’s Reese Witherspoon’s Madeline trying to rescue her marriage from her own adultery, Nicole Kidman’s Celeste having to choose between her sanity and her children, Shailene Woodley wondering if she’ll ever let another man touch her, and Zoe Kravitz’s Bonnie repressing old abuses and recent crimes. (Laura Dern’s Renata is both the most comically outsized of these characters and the most fearsome and battle-weary.)

A show that first weaved a whodunit out of unlikely friendships is now a weekly meditation on female dissatisfactions, sorrows, and rage — where to put them, how to hide them, when to let them loose, and how to clean up afterward. That’s a sequel arguably richer than the original and proof that this new format — not quite TV series, not quite extra-long movie — is evolving into something unique. Name it later; for now, just watch.